84 Definition of Si)ecies. 



rather than of an arboreal vegetation, to gain and keep possession of the 

 prairie soil, aided ])erhaps by fires and other local causes. 



Tliese views of mine may contain erroneous suggestions. I have 

 had no special means to examine soils, or compare wide extents of 

 prairie regions with each other. I arrive at my own conclusions from 

 simple observations of the prairies in th'is part of the State. I am 

 satisfied that no one theory j^et advanced, as to the origin and forma- 

 tion of the prairies, will account for all their phenomena, even in this 

 limited portion of the State. Combined causes, operating with different 

 degrees of force, in different parts of the great prairie regions of the 

 countr}' — sometimes one cause predominating, sometimes another, and 

 sometimes all together — are more in harmony, it seems to me, with 

 the effects left for our observation. 



Definition of Species. 



Wallace says: "In estimating these numbers (of Papilionidce) I 

 have had the usual difficulty to encounter, of determining what to con- 

 sider species and what varieties. The Malayan region, consisting of a 

 large number of islands of generally great antiquity, possesses, compared 

 to its actual area, a great number of distinct forms, often, indeed, dis- 

 tinguished by very slight characters, but in most cases so constant in 

 large series of specimens, and so easily separable from each other, that 

 I know not on what principle we can refuse to give them the name 

 and rank of species. One of the best and most orthodox definitions 

 is that of Pritchard, the great ethnologist, who says, that ' separate 

 origin and distinctness of race, evinced by a constant transmission of some 

 characteristic pecidiarity of organization' constitutes a sjiecies. Now, 

 leaving out the question of ' origin,' which we can not deterniine, and 

 taking only the proof of separate origin, ' the constant transmission of 

 some charactei'istic pecidiarity of organization,' we have a definition 

 which will compel us to neglect altogether the amount of difference be- 

 tween any two forms, and to consider only whether the difierences that 

 present tliemselves are ^^ermanent. The rule, therefore, I have en- 

 deavored to adopt is, that when the difference between two forms in- 

 habiting separate areas seems quite constant, wdien it can be defined in 

 Avords, and when it is not confined to a single jjeculiarity only, I have 

 considered such forms to be species. When, however, the individuals 

 of each locality vary among themselves, so as to cause the distinctions 

 between the two fi)rms to become inconsiderable and indefinite, or 

 where the differences, though constant, are confined to one particular 

 only, such as size, tint, or a single point of difference in mai'kiug or in 

 outline, I class one of the forms as a variety of tlie other. 



